Aircraft are subject to the buildup of ice, frost and/or snow on their surfaces while the aircraft are on the ground during cold weather in the presence of relatively high humidity, rain and/or snow. Ice and snow form strong bonds to metal surfaces. Once a strong bond is formed between ice and metal surfaces, it takes a significant period of time and a high volume of deicing fluid to melt the snow and ice and remove them from the surface. Because the buildup of ice, frost and/or snow on the aircraft surfaces can adversely affect the functioning of the aircraft, such buildups should be removed from the airplane, and aircraft surfaces must be free from contamination before takeoff.
Deicing solutions, such as those based upon ethylene glycol and water, have been used for many years to remove ice, frost and/or snow from aircraft surfaces. These deicing compositions can effectively remove the buildup of ice, frost and/or snow from the surfaces, but since they have low viscosities and have weak interfacial interactions with aircraft surfaces, they do not form a film and stay on the aircraft surfaces. Thus, they provide limited protection against the formation of further frost and ice or accumulation of snow on the surfaces. Furthermore, conventional deicing fluids do not decrease the bonding strength of ice on the metal aircraft surfaces, once the snow and/or ice build-up occurs. This makes the deicing a long and high volume deicing fluid consumption process.
Anti-icing fluids are those that will prevent the formation of frost and ice or the accumulation of snow on surfaces over extended periods of time. The use of anti-icing fluids would be particularly desirable for overnight frost protection and for use during bad weather operation to accomodate substantial delays between the time the aircraft is deiced and takeoff.
One approach to formulating anti-icing fluids is to add thickening agents to deicing fluids. The thickening agents are intended to increase the viscosity of the fluid and thereby reduce its tendency to run off the surfaces of the aircraft. A number of thickened fluids are commercially available in the world. These include Kilfrost Anti-icing Barrier Compound-3, Hoechtst A.G. Aircraft deicing Fluid 1704, and SPCA Aircraft Deicer 84, Union Carbide Corporation UCON (TM) Anti-icing Fluid E, an ethylene glycol-based fluid containing polyacrylate, Union Carbide Corporation Aircraft Deicing fluid 251, a propylene glycol-based product, which is a non-Newtonian fluid containing a crosslinked polyacrylic acid as a thickener, as well as those fluids disclosed in U.S. Pat. No. 4,954,279.
An anti-icing fluid must meet a number of criteria. One of the most important criteria is that the anti-icing fluid be easily removed from the surfaces of the aircraft during its takeoff. This ability to be released from the aircraft surfaces must exist even if no precipitation occurs after the application of the anti-icing fluid and some of the fluid evaporates. Moreover, the fluid should have special rheological properties in that it must have sufficient viscosity to be retained on the aircraft surface while the aircraft is at rest, yet require little force for it to be removed from the surface. Another important consideration in providing an anti-icing fluid is that the fluid should be capable of being applied using conventional spraying devices. Since these spraying devices can subject a thickened fluid to stress, loss of rheological properties is often experienced during the spray application of anti-icing fluids using a cross-linked polyacrylate thickener.
Another concern when using thickened fluids for anti-icing agents is the stability of the fluid during storage. For example, if an anti-icing fluid tends to gel during storage, the fluid, when applied, may be too viscous for effective removal from the aircraft or, if the gelling results in phase separation, the fluid may lose viscosity and thus not provide the desired anti-icing properties.
In addition to the foregoing requirements, good deicing and anti-icing agents should induce no appreciable erosive or corrosive activity, particularly towards aluminum, non-ferrous metals, steel, glass and acrylic sheeting. The anti-icing fluid should also be an effective deicer, resulting in rapid and complete thawing of ice, snow and frost, and it should provide a coherent liquid film after application to the aircraft surfaces, i.e., it should wet the surfaces with which it comes in contact. Most advantageously, anti-icing fluids should be environmentally acceptable, provide minimum toxicity to plants and animals, and provide little risk of flammability.
Tests using commercially available anti-icing fluids on a simulated segment of an aircraft wing indicate that anti-icing fluids can result in appreciable lift loss as compared to deicing (unthickened) fluids. Accordingly, substantial room for improvement in anti-icing fluids for aircraft exists.
U.S. Pat. No. 2,373,727, discloses compositions to prevent or reduce the formation and/or accretion of ice on aircraft surfaces. The compositions comprise a jelly base consisting of gelatinous material and glycol or glycerol or homologs thereof or polyglycols or polyglycerols, e.g., diethylene glycol or tri-ethylene glycol, and from 5 to 20 percent by weight of a liquid which is immiscible with ice or water and remains liquid at temperatures down to 10.degree. F. The immiscible liquids are disclosed to be mineral, vegetable and animal oils, including essential oils such as petroleum lubricating oils, castor oils and pine oils, synthetic oils, the water-immiscible alcohols and similar liquids. The patentees state that it is necessary to add a dispersing agent such as alkali metal or organic-based soaps or fatty acids or oleic acid, napthanates, sulfonic acid salts, commercial soaps or monhydric alcohols. The jelly base may be prepared from any suitable material such as gelatin, glue, soap or gum which forms a heat-reversible, jelly-like mass with the glycol or glycerol.
U.S. Pat. No. 4,358,389 discloses anti-icing compositions that contain 0.05 to 1.5 percent by weight of a particular cross-linked polyacrylate, glycol, water, 0.05 to 1 percent by weight of a water-insoluble component comprising a mixed base mineral oil comprising paraffin hydrocarbons and naphthenes, 0.05 to 1 percent by weight of a surface active agent comprising alkali metal alkylaryl sulfonate, corrosion inhibitor and a quantity of at least one alkaline compound selected from the group consisting of alkali metal carbonates, bicarbonates or hydroxides and amines, such that the pH of the composition is between about 7.5 and 10. The patentees disclose at column 5, line 54, et seq.,
"In addition to the components . . . , the agent . . . can also contain appropriate additives, preferably anti-oxidants and polysaccharides (gums) in effective quantities (gums are additional thickeners) . . . It has been found that polysaccharides have an advantageous effect on the rheological properties of crosslinked polyacrylates, particularly those having viscosity values in the lower range of the viscosity limits indicated above, that is, within the range from about 1000 to 5000 mPas. Preferred polysaccharides are those of the type of high molecular xanthan gum."
The patentees disclose suitable corrosion inhibitors to include those belonging to the group comprising inorganic metal salts, alkali metal salts of fatty acids, monoalkyl amines and dialkyl amines optionally alkoxylated--and salts thereof, alkanol amines--optionally alkoxylated and salts thereof, esters of phosphorus acid or of phosphoric acid, and triazoles.
German Patent application 3,143,059 published May 5, 1983, discloses an anti-icing composition using a cellulose derivative thickener to provide a solution having a viscosity of 100 to 5000 mPa. sec at 20.degree. C. The composition contains surfactant which is an alkali alkaryl sulfonate or oxyalkylated fatty alcohol, corrosion inhibitor and alkali carbonate, alkali hydroxide or amine to give a pH of 7.5 to 10. The application states that the composition may contain other thickeners, particularly polyacrylate or xanthan gum.
An anonymous disclosure in Research Disclosure, Apr. 1985, No. 25246, entitled "Aircraft Deicer," describes aircraft anti-icer concentrate to which water and/or glycol can be added. the anti-icer concentrate comprises water and an antifreeze as the two main components; up to 5 percent by weight of a thickener from the group comprising cellulosic derivatives such as alkyl, hydroxy alkyl and carboxy alkyl, ethers or mixed ethers of cellulose, cross-linked or non-crosslinked polyacrylates, xanthan gum, starch or starch derivatives and guar, it being possible for all except for the polyacrylates also to act as co-thickeners; an active amount of a tenside, optionally in combination with a water-insoluble compound, the tenside being selected from the group comprising polyalkylene glycols, polyalkyl oxalates of alcohols, carboxylic acids, amines or amides, olefin sulfonates, fatty alcohol sulfates and alkyl benzene sulfonates, and the water-insoluble compound being selected from the group comprising fatty alcohols, fatty acid esters, fatty acid alkanolamides, mineral oils, alkyl phenols and alkyl benzenes; customary corrosion inhibitors, such as, for example, carbonates, phosphates, silicates, amines, ethanolamines and benzotriazoles; and sufficient alkali that the pH value of the agent is within the desired alkaline range.